1. Field of the Invention
The present invention relates to a preparation process of aliphatic polyester which is a useful biodegradable polymer as a substitute of medical materials and general purpose resins, by direct-dehydration polycondensation.
2. Description of the Related Art
A biodegradable polymer of aliphatic polyester, particularly aliphatic polyester which is represented by polylactic acid and can be prepared from an aliphatic hydroxycarboxylic acid having a carboxyl group and hydroxyl group in the same molecule (hereinafter referred to as polyhydroxycarboxylic acid) is excellent in mechanical, physical, and chemical properties, also has a biodegradable property and can be degraded in the natural environment without causing pollution finally to water and carbon dioxide by the action of microorganisms. Consequently, biodegradable aliphatic polyester has recently has focused attention as a medical material and a replacement for general purpose resins in view of environmental protection and in other various fields of use, and is now expected to have a large expansion of demand in future.
It has been known in the preparation process of aliphatic polyhydroxycarboxylic acid that a high molecular weight polymer can be usually obtained in the case of lactic acid and glycolic acid by dimerizing aliphatic hydroxycarboxylic acid through dehydration and successively subjecting the resulting cyclic dimer to ring-opening melt polymerization in the presence of various catalysts. The process requires much labor and cost for preparing the cyclic dimer, lactic or glycolide, and is thus unfavorable in economy. Further, some kinds of aliphatic hydroxycarboxylic acid do not form a cyclic dimer and the process cannot be used in such cases.
On the other hand, several processes for preparing aliphatic polyhydroxycarboxylic acid by direct dehydration process from aliphatic hydroxycarboxylic acid or an oligomer of the same have been disclosed (Japanese Laid-Open Patent SHO 59-096123 and 61-028521). However, the polymer obtained by these processes has an upper limit of inherent viscosity at around 0.3 dl/g, does not have satisfactory mechanical properties, and cannot be applied to some uses and objects. Consequently, the preparation process of aliphatic-polyhydroxycarboxylic acid having sufficient mechanical properties is restricted to the ring-opening melt polymerization process of cyclic dimer and the composition of the resulting polymer is also limited in the present state of the art.
The direct polymerization of lactic acid, glycolic acid and other aliphatic hydroxycarboxylic acid is a successive reaction similar to an esterification reaction of dibasic acid and aliphatic polyhydric alcohol. The molecular weight of the resulting polymer increases with the reaction time. The water molecule generated in the reaction has an action for decreasing the molecular weight of the polycondensate by an reverse reaction due to hydrolysis action. As a result, it has been required in order to obtain a high molecular weight polymer to efficiently remove the generated water from the reaction system. Methods which can be used for removing water include increasing the stirring velocity in the reaction, enhancing the extent of reduced pressure, and introduction of an inert gas into the reaction system in order to evaporate water. However, these methods are restricted because the viscosity of the reaction system remarkably increases with increase in the molecular weight.
The preparation process of a high molecular weight polyester from an aliphatic polyhydric alcohol having two or more hydroxyl groups and an aliphatic polybasic acid having two or more carboxylic groups is, as well known in the art, on the basis of the deglycolation reaction of low molecular weight polyester having terminal hydroxyl group. Consequently, the concentration of the terminal group remarkably decreases with increase in the molecular weight and leads to limit the molecular weight. The tendency is markedly found on aliphatic polyester in particular.
For example, in the preparation of high molecular weight aliphatic polyester by conventional deglycolation reaction under reduced pressure, it is observed that the molecular weight once reaches a maximum and then turns into reduction. Conventional deglycolation reaction has been difficult to provide aliphatic polyester having a molecular weight sufficient to form a tough film. In other words, aliphatic polyester obtained by the process could not provide a film having practical utility.
As to a process for preparing aliphatic polyhydroxycarboxylic acid and other aliphatic polyester, efficiently in industry, with ease and at a low cost, U.S. Pat. No. 5,310,865 has disclosed a process for preparing high molecular weight aliphatic polyhydroxycarboxylic acid by heating aliphatic hydroxycarboxylic acid in an organic solvent in the presence of a catalyst, treating the distilled solvent with a drying agent and returning the treated solvent to the reaction system. Further, U.S. Pat. No. 5,401,796 has disclosed a process for preparing high molecular weight aliphatic polyester by heating aliphatic polyhydric alcohol and aliphatic polybasic acid in an organic solvent to progress a dehydration polycondensation reaction, distilling out generated water together with the organic solvent, making the distilled solvent into contact with a drying agent, and returning dehydrated solvent to the reaction system.
In these preparation processes, the removing method of generated water which is an important factor for the progress of the dehydration polycondensation reaction has adopted a system for distilling water-containing solvent under reflux, treating the distilled solvent with a drying agent, and returning again the dehydrated solvent to the reaction system.
However, in order to prepare aliphatic polyhydroxycarboxylic acid in industry, further improvement has been required for the step of removing the generated water. When using a drying agent such diphosphorus pentoxide; calcium hydride, sodium hydride, lithium aluminum hydride and other metal hydrides and sodium, lithium and other alkali metals, the drying agent which reacts with moisture requires labor for reactivation and is not adequate for practical use. On the other hand, the drying agent which adsorbs moisture is ease in handling, can be reactivated, and thus is adequate for practical use. However, molecular sieve adsorbs organic compounds. A solvent which evaporates in the course of the polymerization reaction contains other associately evaporated compounds such as aliphatic hydroxycarboxylic acid, lactide, glycolide and other cyclic dimer which are derived from aliphatic hydroxycarboxylic acid, aliphatic polyhydric alcohol, aliphatic polybasic acid and an oligomer of the same. When molecular sieve is used as the drying agent, these compounds are adsorbed to molecular sieve.
Further, the regeneration temperature of molecular sieve is 200-300.degree. C. Thus, absorbed compounds to molecular sieve are modified and deteriorated. As a result, repeated use of the drying agent contaminates modified ingredients into the polymerization system and has impaired product quality, color tone in particular.